Parathyroid hormone-related protein (PTHRP) frequently causes bone resorption and hypercalcemia in patients with cancer. PTHRP is likely also important in normal bone physiology. The broad objectives of the present proposal are (1) to develop a more sensitive assay for PTHRP, and (2) to investigate paracrine roles of PTHRP in bone. Specifically, during the first visit (6 mo) it is proposed to develop an immunochemiluninometric assay for PTHRP. The signal antibody of an existing, Yale-developed immunoradiometric assay (IRMA) for PTHRP will be tagged with an acridinium ester; following incubation of capture antibody, sample, and signal antibody, photons emitted upon stimulation with peroxide will be quantitated in a luminometer. Following a trip back to Yale to prepare additional affinity-purified antibody reagents for the assay, it is proposed to return for a second visit (6 mo) to investigate paracrine roles of PTHRP in bone-derived cell populations. Specifically, the effects of PTHRP on the induction of antigen expression and calcitonin responsiveness of the osteoclast precursor cell line FLG 29.1 will be determined. In addition, levels of PTHRP will be quantitated, using the new chemiluminescent assay, in conditioned medium from both osteoblast-like and vascular endothelial cells which have been shown to affect maturation of FLG 29.1 osteoclasts.